When Is It Too Late to Fertilize Corn?

Written by Rachel Sim, with expertise provided by Erin Doran, Technical Agronomist.

Around three weeks after the plant has emerged, it enters V6 and the stem begins growing rapidly. Sufficient nutrient supply is critical at this time because the plant’s roots are now large and deep enough to take up the higher amount of nutrients required for this growth spurt.

Critical Growth Phases for Fertilizer Application

“Corn takes in the most nitrogen between V6 and VT during vegetative stage, but phosphorus and sulfur are also critical at this point,” says Erin Doran, Technical Agronomist at Sound. ​“It’s around this growth stage where the plant switches from determining how many rows around the ear there will be to how many rows long it will be, which are the two biggest yield determinants in corn.” 

The VT stage is a major cut-off for fertilizing, in part because the corn is simply getting too tall for machinery to get across the field, but also because it begins to rely more heavily on the nutrients it’s already taken up. 

“Once the field gets to tassel and the reproductive stages kick in, the plants mostly use what they’ve already uptaken for the season, relying on its nutrient reserves,” says Erin. ​“But the tricky part is that if you’re deficient in any one nutrient at any time throughout the season, some other nutrient will be tied up too.” 

In the soil, micro- and macronutrients interact with each other to impact plant availability; excessive levels of one nutrient can make others more difficult for plants to access. For example, boron, copper and potassium can be challenging for plants to access if nitrogen levels are too high. Soils where nutrient levels are balanced and in harmony support increased yield, improved plant health and better ROI. Growers can use tools like Mulder’s Chart to help them identify the best way to balance their soils, taking into consideration additional factors like climate, soil type and management strategies. 

Depending on weather, climate, soil conditions and operational factors, growers may apply their fertilizer in the fall ahead of the next season or in the spring before planting. Since the spring is often busy, if conditions are right, fall applications can save growers time and effort. Phosphorus is neither very mobile nor prone to volatilization, although it can be carried away in runoff and is prone to soil tie-up. In colder regions, low temperatures can inhibit tie up, making fall applications an efficient option for growers. 

Erin is in Iowa, where most growers do fall fertilizer applications. ​“Nearly 90% of fertilizer here is put out before planting; only a small percentage goes out after,” she says. ​“Under normal conditions, the fertilizer won’t go away until the plant is there, and the plant won’t use it until it’s needed.” 

In the fall and winter, Erin says growers will put out anhydrous, monoammonium phosphate (MAP) and diammonium phosphate (DAP).

“The soil is cold enough that the nutrient cycle is dormant, so nitrogen will stay there in its anhydrous form until the soil warms up. Phosphorus and potassium are going out in granular form because as long as the ground temperature is below 60℉, they won’t start breaking down until it gets warmer either,” she says. 

Risks of Late Nitrogen Applications

Nitrogen is particularly prone to loss through leaching, nitrification, or volatilization when applied months ahead of when plants need the nutrient. Colder weather is key since low temperatures reduce nitrification, especially when paired with a stabilizer or nitrification inhibitor. Dry soils are also critical since lingering wetness and rain increase nitrogen loss. 

“Phosphorus and potassium aren’t very mobile in the soil, so the concern is focused on making sure soil is balanced so they don’t get tied up in the soil,” says Erin. ​“Nitrogen is the one that’s very mobile within the soil: rain can wash it out of the soil and make us lose a high percentage, drought makes it so it won’t convert to a plant available form.

Generally, organic fertilizers release nutrients more slowly than synthetic fertilizers and can include things like manure and crop residues. Because organic fertilizers break down more slowly, they can be a good choice for early application. When using manure, there is some potential for nutrient loss through nitrification, volatilization and in runoff, but incorporating manure into the soil and applying after temperatures drop can reduce the risk. 

In season, growers can either sidedress or topdress additional fertilizer if needed. Both sidedressing and topdressing need to be watered into the soil, either through cooperative weather or a grower’s own access to irrigation. Without rain, topdressed nitrogen can be lost to the environment through volatilization and it is a less precise application method. While sidedressing also requires watering in, the nitrogen is generally placed in the soil, making it more stable and less prone to loss. 

Nitrogen and Side-Dressing

Sidedressing is one way growers can meet the corn’s increased nutrient demand in the middle of the season. Low nitrogen from V6 through V10 can result in significant yield drag; growers may plan to sidedress additional nitrogen mid-season or try to make up for nitrogen loss as the result of unexpected weather conditions earlier in the season. Growers are also able to be more precise and targeted with sidedressing, applying what the plant needs and no more.

Impact of Heavy Rainfall on Nitrogen Loss

Warmer fall or winter weather than anticipated can speed up nitrogen loss, but so can heavy rain both before or in season, leaving growers looking for ways to ensure sufficient nutrition is available to their corn crops. That’s what happened to growers in Iowa in 2024, Erin says. 

“We had a very weird season in 2024; it was super wet early in the season, which means we lost a lot of our nitrogen reserves that were in the soil,” says Erin. ​“Growers needed to come back to side dress to finish the season out, but then we went into a drought.” 

The choice of corn fertilizer is driven by a number of factors depending on a grower’s individual operation and geographic location. Understanding their soil’s unique needs is critical for growers when developing a nutrition plan. Factors like pH can reveal which nutrients will be more or less available to the plant, and knowing what is already in the soil can help guide more efficient nutrient use.

Conditions can vary across a single field, so periodically grid testing fields can help growers remediate specific areas ahead of planting. In-season tissue testing is also important; not only will it make it easier to catch nutrient deficiencies early and potentially remediate, but testing in troublesome areas can let growers know if the issue is in the soil or if nutrients are not plant-available.

How Healthy Soils Help Corn Growth

In-season fertilizer may be applied to help carry the crop across the finish line since nutrition demands don’t disappear even after tassel, but at this stage, corn’s height can make getting machinery across the field challenging and flying on fertilizer is imprecise. This is where healthy and active soils can make a huge difference. 

Over the millenia, plants and soil microbes have evolved unique, mutually beneficial relationships. In exchange for some of the carbohydrates plants create through photosynthesis, phosphorus solubilizing and nitrogen fixing microbes provide growers with access to free, in-season nutrition. Arbuscular mycorrhizal fungi (AMF) not only act as intermediaries between plants and soil microbes, they also extend roots’ reach and surface area, improve soil texture and water retention, and even provide some protection against certain pathogens. Sufficient soil organic matter increases water-holding capacity, improves drainage, and supports an active soil microbiome. Healthy soils support the crop throughout its development.

To help stimulate and boost soil’s native microbes and provide late-season nutrient support the crop needs, a microbiome activator like Sound’s SOURCE® is a good option for growers. SOURCE has a large application window for corn — from V4 to R3 — helping growers meet their nutrition needs throughout the season. 

“The tricky part with nitrogen especially is that there’s no silver bullet number for application,” says Erin. ​“It’s very dependent on the weather and how healthy your soil is. This is where SOURCE has some long-term effects benefitting soil health. The more organic matter it can break down, the more nutrients can be made available to the crop. SOURCE activates the soil microbes that take atmospheric nitrogen and turn it into a form that plants can use.” 

It’s not uncommon for agricultural soils to have low AMF populations, in part because intense tilling, fertilizing, and fungicides can be particularly hard on fungi. Adding in the powerful mycorrhizal inoculant BLUEPRINT^TM can help growers support or replenish the AMF in their soil, and it’s particularly effective when used with SOURCE. Together, the products work synergistically: SOURCE activates all microbes in the soil, both bacteria and fungi, and BLUEPRINT boosts soils’ AMF populations to increase plants’ ability to access the nutrition microbes provide.

Products like SOURCE and BLUEPRINT offer growers flexibility in meeting corn’s nutrient needs throughout the season.

“SOURCE is going to help take what you have already applied to the soil and make it more available to the plant. In an extreme drought situation or some really heavy manure fields, the form it was put out, the soil ties it up and holds on to it and it never gets to the plant,” says Erin. ​“That’s what SOURCE is helping with. It’s getting what’s there to the plant instead of letting it go somewhere else.”

To implement these fertilization strategies with cutting-edge tools like SOURCE®, visit How to Buy for more information.